This invention relates to a developing device for an electrophotographic copier for applying developing agent to an electrostatic latent image formed on a photoreceptor by exposure to an image forming beam of light.
In order to explain the background of the present invention clearly, an example of a prior art developing device for an electrophotographic copier with a photoreceptor is shown at A in FIG. 1 with its developing tank 1 provided with a stirrer roller 3 for stirring developing agent 2 (toner and carrier), a developer roller 6 for applying the developing agent 2 onto the photoreceptor 4, a doctor blade 7 for adjusting the amount of developing agent 2 to be attached to the photoreceptor 4 and a toner concentration sensor 8 for detecting the toner concentration in the developing agent 2. As shown both in FIGS. 1 and 2, the developer roller 6 is comprised of a magnet structure 11 composed of ferrite magnets with 3-5 poles 9 and a cylindrical support rod 10, a magnet supporting shaft 11a for supporting this magnet structure a sleeve 12 which movably engage externally around the magnet structure 11 and a sleeve shaft 12a for supporting the sleeve 12. The magnet structure 11 is affixed through its supporting shaft 11a to the developing tank 1. When this developing device A is in operation, the sleeve 12 is caused to rotate and the amount of developing agent 2 attached to the sleeve 12 by the force of the magnet 9 is controlled by the doctor blade 7. In FIG. 2, numeral 13 indicates a bearing for supporting the sleeve 12 rotatably by the developing tank 1, numeral 14 indicates driving means for rotating the sleeve 12, numeral 15 indicates its driver motor and numeral 16 indicates a gear for communicating the driving power of this driver motor 15 to the sleeve shaft 12a. In FIG. 1, numeral indicates a toner supply roller disposed above the developing tank 1 for rotating in response to a signal from the toner concentration sensor 8 to supply toner 2a from a toner box 18 into the developing tank 1.
Of the developing device A described above, the sleeve 12 is an expensive component but since it covers the entire circumference of the magnet structure 11 as shown in FIG. 1, its required area is large and hence increases the cost of the development device A as a whole.